"The Doomsday Machine"[TOS2] Episode Synopsis

From outside the galaxy, a conical weapon-ship
appears, and enters the galaxy. Miles in length, protected by a
solid neutronium hull and a powerful energy dampening field, the device is armed
with an antiproton beam capable of chopping planets to rubble.

The weapon-ship is an automated device, designed as
a planet-killer. It is intended to cut apart entire worlds and consume
some of the material, fueling its total conversion drive. Several systems
are destroyed in this manner as the device cuts a path that will carry it
through some of the most populous worlds of the Federation.

The Constitution Class Starship USS Constellation,
NCC-1017, commanded by Commodore Matthew Decker, finds some of these destroyed
star systems. However, heavy subspace interference prevents the ship from
contacting Starfleet to advise. The ship continues to investigate,
eventually coming up on system L-374, near the Rigel system. There, they
note that the fourth planet is in the process of breaking up, and approach.

The Constellation finds the planet-killer in the
process of chopping the fourth planet to bits. Their approach, however,
brings them into the defensive sphere of the device, and the ship comes under
attack. The deflector shields, already strained due to the dampening
field, are quickly overwhelmed, and soon the planet-killer's weaponry begins
goring holes into the ship, melting and scorching the hull.

Decker and the Constellation attempt to escape
toward the inner system, but the damage is done. The ship's bridge is
uninhabitable, the generators are damaged, the phasers exhausted and useless,
the impulse controls are burned out, and the starship's antimatter supply has
been rendered inert. She's operating on reserve energy banks
only. Unable to escape, unable to fight, and unable to call for
help, the Constellation is a sitting duck for the approaching weapon-ship.

Worse yet, the life support systems are barely
operable, and the air filtration system is down. In short, the ship was
nothing more than a target, and one which could only barely sustain her crew, if
at all. Decker thus gives the order to abandon ship, and the 400
crewmembers are beamed to the relative safety of the third planet of
L-374. Commodore Decker remains behind.

Suddenly the weapon-ship is upon him, attacking the
Constellation once again. The transporters are destroyed, and the ship
adrift. But to Decker's horror, the device loses interest in the wrecked
Constellation, and instead takes advantage of its position near the third planet
to begin that world's disassembly. His crew calls for beam-up, but
it's too late . . . the transporters are gone, the ship is immobilized, and all
Decker can do is watch as the planet to which he sent his crew for refuge is
destroyed beneath their feet.

Unbeknownst to Decker, the USS Enterprise was close
enough to pick up the Constellation's weak, badly-garbled distress call.
As the Enterprise travels toward the source of the call, they also discover
destroyed solar systems. Eventually, the ship reaches L-374, finding that
only the two inner planets of the system remain. Locating the
Constellation via her disaster beacon, Kirk and a landing party board the ship,
finding a distraught Decker and assessing the damage to the ship. Kirk
prepares to take the Constellation in tow.

As McCoy and Decker beam over, however, the
weapon-ship approaches. Kirk orders the Enterprise to beam the teams
from Constellation aboard, but the device attacks the Enterprise, knocking out
the transporter and damaging the comm systems. The Enterprise
escapes, eventually getting outside the machine's defensive sphere, but before
the ship can circle back to pick up the crewmen on the Constellation, Decker
assumes command. The loss of his crew having driven him mad, Decker
attacks the weapon-ship, engaging in a phaser barrage at point-blank
range. But because of the neutronium hull, the phaser fire simply bounces
off, and because of the machine's field, the ship's deflector shields strain to
the limit even when the machine isn't firing.

Meanwhile, Kirk and the engineering team are able
to restore severely limited impulse power, sensors, and even a phaser bank on
the Constellation. Thus, when Decker's futile attack has resulted in
shield failure, hull breaches, generator failures, and the loss of warp power
aboard the Enterprise, and with the weapon-ship tractoring the Enterprise into
its giant maw, Kirk is able to use the Constellation as a
distraction. The Constellation is barely mobile, nearly impossible
to control, and of limited combat utility, but its motion and weapons fire is
sufficient to draw the planet-killer's attention away from the
Enterprise. Decker then returns the favor, drawing the device's
attention back toward the Enterprise and leading it away from the Constellation.

Communication is restored between the two Starfleet
vessels, and Kirk orders Decker relieved. Spock takes command,
ordering Decker to Sickbay. But, Decker overwhelms the security guard
escorting him, and soon steals a shuttlecraft. Despite the entreaties of
Kirk and Spock, Decker pilots the shuttle on a suicide run into the belly of the
beast.

What is at first thought to be a senseless death
becomes the key to destroying the Doomsday Machine. The explosion of
the shuttlecraft produces a minute power drop in the device. Kirk has the
damage control teams beamed back to the ship, and orders Scotty to rig the
impulse engines, now capable of almost one-third impulse, to explode at Kirk's
command. His plan is to use the impulse engines, each capable of
generating a 97.835 megaton explosion when overloaded, to try to destroy the
planet-killer from the inside. Scotty sets the 30-second delay and,
per Kirk's order, beams to the Enterprise. Kirk approaches the
planet-killer at over 50 kilometers per second relative speed, then within 15
seconds slows his approach to approximately 10km/sec. Kirk thereby pilots the
Constellation straight into the maw of the weapon-ship, the half-operable
transporter beaming him off at the last moment before the detonation.

The blast completely deactivates the Doomsday
Machine, rendering it inert.